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When hummingbirds fly unfriendly skies

Hovering behind a fat obstacle like a tree trunk on a windy day may have costs

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8:05pm, March 25, 2014

HOVER MASTER  An Anna’s hummingbird can sip nectar in fairly bumpy air but may have to calculate when a meal isn’t worth the energy expenditure of fighting turbulence.

Magazine issue: Vol. 185 No. 8, May 3, 2014
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Swirling air can make hummingbirds work harder to hover, but only when the air’s vortices open wider than a bird’s wing.

The first measurements of how much a flying animal’s metabolism revs up when coping with turbulent air come from five Anna’s hummingbirds (Calypte anna) that Victor M. Ortega-Jimenez of the University of California, Berkeley and his colleagues tested. In a wind tunnel, the hummingbirds hovered at a feeder downwind from a cylinder of varying size. Buffeted by vortices of air whipping off slim cylinders (2 or  4 centimeters in diameter), the birds held their position without needing extra oxygen even with wind speeds of 9 meters a second, or about 20 miles per hour.

But when researchers used a 9-centimeter-wide cylinder, vortices widened to 173 percent of wing length. This time hummingbird metabolisms increased some 25 percent on average — even at gentler wind speeds of 3 and 6 meters per second. The hummingbirds relied on asymmetric tail and wing motions to hover in place, the researchers report March 26 in Proceedings of the Royal Society B.

TOUGH AIR  An Anna’s hummingbird can cope with modest breezes and turbulence but reaches a point where vortices spinning off an obstacle require strenuous work to hover. Credit: V.M. Ortega.

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